Mechanical Characterization
of
Materials
Aim and the Objectives
• Aim: To understand why characterization of materials is important and
required.
And how the outcome of this can help us to improve the materials
properties and develop new materials
• Objectives:
 Understand and differentiate between different testing methods,
 Underline and discuss why different materials require different testing
methods,
 To be able to evaluate pros and cons of each method and design own
method for a particular need.
Commonly Used Mechanical Testing Techniques
Mechanical testing
Strength Measurement
(Static)
Dynamic
Tension (Tensile)
Testing
Toughness
Testing
Compression
Testing
Bending
(3 or 4 point)
Fracture Toughness
Testing
Notch Toughness
Charpy
Izod
Time
Dependant Behaviour
In- Plane Impact
(Drop Impact Testing)
Fatigue
Testing
S/N behaviour
Fatigue crack growth Rate
Creep Testing
Stress Relaxation
Plus Effect of temperature and
environment
• What is Mechanical Testing:
To understand and describe how materials deform
(elongate, compress, twist) or break as a function of
 applied load,
 time,
 temperature and other conditions, standard test
methods and standard language (terminology) are
needed.
If we can identify how damage initiates and
propagates within the structure, we can improve the
structure through processing and therefore develop
new materials.
Tension Testing
•
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The tension test is commonly
employed to determine:
Young’s modulus (E),
Yield strength (sy),
Ultimate Tensile Strength (UTS),
Percent elongation (%EL)
Percent reduction of area (%AR).
Specimens are usually round or flat
s = P/A0
where A0 is the original crosssectional area in the gage section, and
P is the applied load.
Fig. 1. Tensile specimens.
Testing machine:
The testing machine is (generally)
a screw-operated tension–
compression machine
with a movable upper crosshead
and a fixed lower base (Fig. 2). A
load cell in series with one of the
grips is used to monitor the load P
on the specimen.
Fig. 2. Tension machine.
Extensometer: A displacementmeasuring device that is attached directly
to the specimen in its gage length, is
used to determine the
axial strain
= Dllll0
where l0 is the original length and l is the
current length.
Strain Gauges: Strain on the specimen
can be measured with strain gauges.
These devices use changes in
•resistance,
•inductance
•capacitance to produce an electrical
signal proportional to strain. Strain gauge
is attached to the surface using glue
therefore both sample and the gauge
deform equally.
Fig. 3.a. Extensometer.
Fig. 3.b. Strain gauge
• Typical stress–strain curves
 For all materials, the slope E of the linear portion of the stress–strain curve for
small strain is a characteristic of the material, called Young’s modulus.
E= 200 GPa (steel)
E= 70 GPa (aluminum)

Young’s modulus is an example of a bulk property of a material which is
determined primarily by the major constituent of the material. Modulus for steel is
about 190–200 GPa regardless of the alloy, cold working or heat treatment.
 All materials have an ultimate strength given by
su= Pmax/A0
where Pmax is the maximum load sustained in tension, and A0 is the original crosssectional area.
 Unlike Young’s modulus the ultimate strength depends strongly on alloy content
and processing variables. Materials, such as piano wire steel and spider webs
(fibrous materials) have incredibly large ultimate strengths.
Strength versus Stiffness
Strength is not the same thing as Stiffness.
Stiffness (E), is concerned with how stiff,
flexible, springy or floppy a material is.
Strength is the force or stress needed to
break a thing.
A biscuit is stiff but weak, steel is stiff and
strong, nylon is flexible but strong,
raspberry jelly is flexible and weak.
The two properties together describe a
solid about as well as you can reasonably
expect two figures to do
Fig. 5. Strength vs. stiffness.
Brittle and Ductile Materials
For a brittle material, the stress–strain
curve is
linear almost to failure.
Therefore the failure occurs before the
yield strength is reached. Glass, highstrength
steels, and some polymers
as (PMMA) exhibit brittle behavior.
For a ductile material, there is a
distinct yield point beyond
which
the stress grows very slowly (if at all)
with strain.
Fig. 4. Brittle vs. ductile behaviour.
 Ductile materials will often neck
prior to failure.
The necking begins when the
ultimate strength has been
reached, at which point the stress
begins to decrease with increasing
strain.
Prior to necking, the deformation
is homogeneous, after necking the
deformation is inhomogeneous.
Fig. 7. Necking
Measures of ductility
Two measures of ductility are used:
Percent elongation and percent reduction of area.
 Percent elongation (%EL) is the observed
nominal strain at failure, expressed in percent:
% El 
l f  lo
lo
100%
Fig. 8. Nonuniform strain in ductile failure.
The original gage length and specimen diameter
must be reported when giving this information,
since the strain at failure is nonuniform

Percent reduction of area (%RA) is given by:
% RA 
Ao  A f
Ao
100%
Percent reduction of area is not
dependent on
original gage length or specimen
diameter; for this
reason, %RA is a preferred measure of
ductility.
Compression Testing
• A compression test is commonly
employed to enhance data for
materials that have been tested in
tension, or to test materials that
are difficult to test in tension, such
as brittle ceramics, building
materials.
 Young’s modulus (E),
 Yield strength (sy), and
ultimate strength are usually
determined.
 Ductility properties, % EL
and % RA are not measured
as ductile material will not fail
in compression.
• Specimens are usually round or
rectangular, and must be fairly
short to avoid buckling.
Fig. 9. Compression machine.
Testing machine:
Most tensile testing machines can be used
to conduct tests in compression by
moving the crosshead in the opposite
direction. Tension grips are replaced by
flat platens.
•
Extensometer:
As in the tension test, an
extensometer is used if any straindependent parameters, such as
Young’s modulus and 0.2%-offset
yield stress are sought. However,
if only ultimate strength is sought,
an extensometer is not needed.
• Typical stress-strain curve:
The behavior of a material in
compression often differs from
that of the same material in
tension.
For ductile metals, it is often
observed that failure will not occur
in compression
 cast iron, which is extremely brittle
in tension but is very ductile in
compression.
Fig. 10. Compression behaviour of
ductile materials
Bending Properties of Materials
Brittle materials, including most ceramics
are difficult to test in tension because:
 Stress concentrations in the fillet
region tend to cause failures near the
grip
 Any misalignment of the axial load
induces bending in addition to tension
 In addition, many ceramics are difficult
to machine therefore the cost of
producing dog bone-shaped specimens
for tensile testing can be prohibitive.
Fig. 11. Tension and compression
stresses arise due to misalignment of the
sample
 3- and 4-point bending tests are
developed for the brittle materials.
 Problems of gripping are minimized as
the stresses are primarily compressive in
the regions of contact with the load and
support rollers (brittle materials are
usually relatively strong in
compression).
• Maximum tensile stresses are developed
along only one edge of the specimen,
and failure usually initiates at the
surface in this region.
 in the 3-point bending test, the
maximum tensile stress occurs at only
one line along this edge (perpendicular
to the plane of loading),
 in the 4-point bending test, the
maximum
tensile stress occurs over an area along
this edge
Bending test will result in a higher
strength than that measured from a
tensile test,
s max 
3PS
2bh 2
3 point bending
s max 
3PS
4bh 2
4 point bending
Fig. 12. 3 and 4 point bending test
configuration
Toughness
Toughness = the ability to absorb
energy up to fracture
Total amount of energy absorption
can be calculated by
the total area under the strainstress curve up to fracture
C'
W   Fd
A
Fig. 13. Brittle versus ductile failure
Notch Toughness Testing
Charpy and Izod
•
•
To test fracture characteristics of
isotropic materials under high strain
rates
Two standard tests, the Charpy and
Izod, measure the impact energy (the
energy required to fracture a test
piece under an impact load), also
called the notch toughness.
Impact resistance of the material is
measured by the movement of the
pendulum after the impact.
From the mass of the pendulum and
difference between the released and
final heights energy absorption can
be calculated
Greater swing of the pendulum
after impact, smaller the energy
absorbed
As temperature decreases a ductile material
becomes brittle.
Alloying usually increases the ductile-to-brittle
transition temperature.
FCC metals remain ductile down to very low
temperatures.
For ceramics, this type of transition occurs
at much higher temperatures than for metals.
In- Plane Impact (Drop Impact)
Testing
Release mechanism
Drop impact test is employed to
assess the impact performance of the
composites (anisotrophic materials).
A projectile is dropped onto a plate
type specimen. The mass and the
height of the impact is kept constant.
Data is produced from force/ time
and force/ displacement curves.
Most easily identifiable parameters
are
•Peak Force (Fmax),
•Total Energy absorbed (E),
•Velocity,
•Deformation.
Impactor
Velocity measurement&
anti-multiple strike
Specimen
mgh 
Impact support
boundary
conditions
1 2
mv
2
Fracture Toughness
For every material has specific critical stress intensity factor
of which if
 KI<KIC the crack is stable and will not grow,
KI=KIC the crack will grow and in most cases the crack growth is catastrophic.
KIC can be calculated by loading the compact tension samples to failure under Pmax,
The crack opening displacement is also monitored.
K I  s 2 r
Stress intensity factor at the sharp crack (notch)
Fatigue:
Failure under fluctuating / cyclic stresses
• Fatigue is a process of initiation and propagation of cracks in a
material due to alternating loading.
• Under fluctuating / cyclic stresses, failure can occur at loads
considerably lower than tensile or yield strengths of material
under a static load.
• Estimated to causes 90% of all failures of metallic structures
(bridges, aircraft, machine components, etc.)
• Applied stresses causing
fatigue may be:
 axial (tension or compression),
 flextural (bending),
 torsional (twisting).
s mean 
s max  s min
2
Ds  s max s min
s amp
(Mean Stress)
(Range of stress)
Ds s max  s min


2
2
s min
R
s max
(Stress amplitude)
(Stress ratio)
S/N curves:
stress-number of cycles to failure
Result of the cyclic test is commonly
plotted as s (stress) vs. N (number of
cycles to failure)
 Low cycle fatigue: high
loads, plastic and elastic
deformation
 High cycle fatigue: low
loads, elastic
deformation (N >105)
Fatigue limit (endurance limit) occurs
for some materials, such as Fe and Ti
allows. In this case, the S-N curve
becomes horizontal at large N. The
fatigue limit is a maximum stress
amplitude below which the material
never fails, no matter how large the
number of cycles is.
In most alloys, s decreases
continuously with N. In this
cases the fatigue properties are
described by:
Fatigue strength: stress at which
fracture occurs after specified
number of cycles (e.g. 107)
Fatigue life: Number of cycles to
fail at specified stress level
Note that hundreds of fatigue tests on a
given material must be conducted to give
enough data to construct an accurate and
statistically meaningful SN curve. Fatigue
testing is therefore time consuming and
expensive.
Figure. Cyclic testing of car suspension
component made from rubber and steel
Crack initiation and propagation
Fatigue failure proceeds in three distinct stages:
Crack initiation: in the areas of stress concentration,
Crack propagation:
Stage I: initial slow propagation along crystal planes with
high resolved shear stress. Has flat fracture surface
Stage II: faster propagation perpendicular to the applied
stress. Crack grows by repetitive blunting and sharpening
process at crack tip. Rough fracture surface.
 Catastrophic failure.
Nf = Ni + Np
Nf → Total number of cycles to failure
Ni → Number of cycles to initiate the crack
Np→Number of cycles to propagate the crack
Stress amplitude, mean stress, frequency and surface finish
are important parameters that affect the fatigue life of a
component.
Creep
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•
•
Creep is the time-dependent strain that
occurs in all materials at constant stress.
In metals and ceramics, creep becomes
noticeable when the temperature reaches
about 1/3 to 1/2 of melting temperature.
In polymers, creep becomes noticeable
when the temperature approaches the
glass-transition temperature.
(which is near room temperature for
many polymers)
Creep mechanisms (such as grain
boundary diffusion, dislocation
movements and bulk diffusion) give rise
to permanent (plastic) strains. When the
stress is removed, there will in general be
some elastic recovery, but the material
will not return to its original state.
A typical strain v time graph
of metal and ceramic.
Secondary state is the most
important as the rate increases
rapidly. Several specimens are
tested at different stress value
to determine the effect of
stress on the strain rate.
Then the strain rate is plotted as a
function of stress to determine the
dependence of the strain on stress
(both axis in log scale)
m gives the indication which
creep mechanism is operating.
Stress Relaxation
In polymer materials the time dependent
deformation is important (even at room
temperature) due to their weak van der Waals
inter-chain forces.
In stress relaxation test, the sample is tested at constant strain
(extension) and materials response is measured in terms of
decrease in the stress as a function of time.